Because people with undiagnosed HIV will not receive treatment and may unknowingly infect others, we propose a mobile computer-based video intervention to increase HIV test rates in high volume urban hospital emergency departments (EDs). EDs offer important points of contact for many of those at greatest risk for HIV. Unfortunately, when ED patients are offered routine HIV testing, most decline. Our proposed intervention builds upon initial findings from a trial our research team conducted with patients who declined HIV testing. The intervention, grounded in the Information-Motivation and Behavioral Skills model (IMB), showed an onscreen physician explaining the importance of HIV testing (to build knowledge and motivation) and modeling a rapid HIV test (to increase motivation and behavioral skill). This brief intervention had a potent effect: a third accepted HIV testing post-intervention. While this preliminary study is highly encouraging, it revealed a number of other critical research questions. First, it remains unclear what intervention component most strongly contributed to patients' decisions to test: the video content or the offer of an HIV test by a computer rather than a person. Second, consistent with the literature, participants indicated a community member disclosing positive HIV status onscreen would increase the proportion of patients who test. Third, results suggest there is individual variation n the extent to which behavior is more strongly influenced by onscreen community members or experts (e.g. physicians). Therefore, the goal of the present study, guided by the IMB model, is to determine how we can refine mobile computer-based interventions to maximize HIV testing rates among patients who initially decline to test in the ED. The present study will use a pilot four-arm randomized controlled design. All participants will use mobile computers to complete a pre-test assessment (socio-demographics, HIV test knowledge, substance use screening). One arm will see video of a physician explaining the importance of HIV testing and modeling rapid testing (similar to the original video). The second arm will view video of a community member who explains testing importance, models testing, and discloses he is HIV positive. A third will have a choice of which of the two videos to watch. A fourth group (control) will not see any video. At the end of the computerized intervention (pre-test/video or pre-test only), onscreen text will ask patients if they would agree to an HIV test. Those who agree will be tested by ED staff. The trial will recruit patients (N=300) aged 18-64 in the ED of St. Luke's Hospital Center in New York City. The facility serves roughly 100,000 patients each year, approximately 45% Black or African American and 40% Hispanic or Latino. The aims of the study are to design, develop, test, and evaluate a refined set of mobile computer-based intervention components, including brief videos. The study's endpoint will be post-intervention HIV test rates. Another aim is to better understand patient experience with the intervention, and elicit staff perspectives on implementation using qualitative methods. Our study will inform scalable interventions for underserved populations nationwide.